Gfind_unwind_table.c:59:8: error: implicit declaration of function

```
In file included from dwarf/Lfind_unwind_table.c:4:
dwarf/Gfind_unwind_table.c:59:8: error: implicit declaration of function
'_Uelf64_valid_object' is invalid in C99
[-Werror,-Wimplicit-function-declaration]
if (!elf_w(valid_object) (&edi->ei))
^
./elfxx.h:42:25: note: expanded from macro 'elf_w'
# define elf_w(x) _Uelf64_##x
^
:97:1: note: expanded from here
_Uelf64_valid_object
```

Build setup:
Ubuntu 18.04
Clang 7.0